In response to a request for declaratory ruling from Engineers for the Integrity of Broadcast Auxiliary Services Spectrum, the FCC Wireless Bureau confirmed that
“After careful review of the issues and the information submitted, we find that when a transmitted emission continues to be frequency modulated and is contained within the bandwidth limits specified for frequency modulation, and when there are no modifications to transmitter circuitry, a change in the emission designator is not necessary.”
Stations have found they can use devices like Nucomm's Analog Coder 2
to convert an ASI or SMPTE 310 stream into an analog signal that can be applied to an existing analog microwave link and decoded on the other end to recover the digital data. The transmitted signal is still FM, and EIBASS demonstrated that the spectra of a signal from an FM video microwave using codecs had the “…signature 'triangular' spectral shape associated with an analog transmission as opposed to the 'rectangular' spectral shape seen with using QAM or COFDM.”
This should be good news to broadcasters using these analog codecs, as conversion of a microwave link from analog to digital is a major change requiring a new public coordination. The interference criteria for digital links are different than that for analog links. I have seen cases where it’s impossible to coordinate digital operation on a frequency previously coordinated for analog use.